1. Field of the Invention
The invention relates to radio frequency communication and more particularly, to voltage controlled oscillators (VCOs) used in radio frequency signal transmission systems.
2. Description of Related Art
Voltage controlled oscillators (VCOs) are a common component in most communications circuits. VCOs typically utilize a variable control voltage input to produce a frequency output. The control voltage input typically may be tuned so that the VCO produces a desired, operational frequency output. Examples of VCOs and the manners in which they are tuned may be found in U.S. Pat. Nos. 5,663,690 and 5,764,109. As these patents show, a typical inductor and capacitor tuned VCO generally requires a number of off-chip components including inductors, capacitors, resistors and varactors. Using off-chip components rather than an integrated circuit (IC) requires more space, increases the power requirements, and adds significantly to the cost of a radio frequency communication system. One problem with known VCOs is that there are few, if any, integrated circuit VCOs that can provide a desired differential output, which is necessary for high performance transceivers, and that can maintain a fixed frequency over all process and environmental variations. Further, there are few, if any, integrated circuit VCOs that can adjust the amplitude of the VCO to optimize power consumption and optimize spectral purity, features that are key to battery operated wireless communication devices.
As such, there is a need for a VCO that can be fabricated with CMOS technology to reduce the cost and space required for typical VCOs. Further, there is a need for an integrated circuit VCO that can provide a differential output, maintain a fixed frequency, and adjust the amplitude for optimum power and optimum spectral purity.